The invention relates to a method and a device for controlling a combustion system and for catalytic removal from exhaust gas of a noxious substance generated by the combustion system, in which the noxious substance reacts chemically with a fluid that is added to the exhaust gas in a metered manner. Such a reaction is, for example, the reaction of nitrogen oxides with a reducing agent such as ammonia to form nitrogen and water, which occurs in so-called deNOx catalytic converters. The invention also relates to a combustion system, in particular an internal combustion engine, which is equipped with such a device.
During the combustion of a fossil fuel in a combustion system, noxious substances such as nitrogen oxides, hydrocarbons, carbon monoxide and sulfur oxides are produced and, at appropriate combustion temperatures, dioxines and furanes as well, which can pass into the environment through the exhaust gas of the combustion system. Such a combustion system can, for example, be a boiler system, a fossil-fuel power plant or an internal combustion engine.
Due to strict legal requirements which limit the exhaust gas quantity of the above-mentioned noxious substances, additional treatment of the exhaust gases is necessary for the above-mentioned combustion systems in order to reduce the noxious substances contained in the exhaust gases. In the past, a number of catalytic converters have been developed for that purpose. For example, the deNOx catalytic converters already mentioned are known for reducing nitrogen oxides. The catalytic converters convert the nitrogen oxides contained in the exhaust gas by using a suitable reducing agent, usually ammonia, according to the method of selective catalytic reduction (SCR), to form environmentally friendly nitrogen and water. The reducing agent or a precursor of the reducing agent, for example urea as a precursor of ammonia, is introduced into the exhaust gas upstream of the catalytic converter, in flow direction of the exhaust gas, and then enters the catalytic converter in a preferably homogeneous mixture with the nitrogen oxides contained in the exhaust gas.
In order to reduce the noxious substances in the exhaust gas of an Otto or spark-ignition engine, noble metal-containing catalytic converters, for example, are known, in which hydrocarbons and carbon monoxide are converted by using nitrogen oxides to form carbon dioxide, nitrogen and/or water.
Furthermore, work is currently under way at many locations on the development of a regulated diesel catalytic converter with which it is to be possible to considerably reduce the nitrogen oxide content in the exhaust gas of a diesel engine. However, due to the high residual content of atmospheric oxygen in the exhaust gas, that is not possible with the noble metal-containing catalytic converter known for the Otto or spark-ignition engine. Instead, the already mentioned deNOx catalytic converter can be used to remove nitrogen oxide from the exhaust gas of a diesel engine. However, since a diesel engine is usually operated with variable load and variable rotational speed, in particular when it is used to provide traction, the quantity of nitrogen oxide generated per unit time is subject to fluctuation within wide limits. The same also applies, of course, to any other combustion system which is used with changing operating conditions.
On one hand, in order to achieve a high rate of precipitation of the nitrogen oxides and, on the other hand, to prevent a slip of the introduced reducing agent, the reducing agent must be metered precisely to the generated quantity of nitrogen oxides through the use of suitable measures in accordance with the stoichiometry of the reaction. An aggravating factor is that the ammonia which is used as a reducing agent is poisonous and even a small concentration constitutes a considerable olfactory nuisance for humans. Precise metering is also necessary with other catalytic cleaning methods in which a noxious substance reacts as a first reactant, with a fluid which has been additionally introduced into the exhaust gas as a second reactant.
Such precise metering of the fluid is associated with a considerable technical outlay since, in addition to monitoring the generated quantity of noxious substance, it is also necessary to additionally take parameters which relate to the operation of the combustion system into account. Those parameters are, for example, temperature and load, parameters which relate to the operation of the catalytic converter, for example catalytic converter temperature and storage capacity, and exhaust-gas parameters, for example pressure and temperature. A method for the controlled introduction of a fluid into a nitrogen oxide-containing exhaust gas, which takes the afore said parameters into account, is known from European Patent EP 0 697 062 B1, corresponding to U.S. Pat. No. 5,628,186. It is disadvantageous that the metering system which is necessary for the introduction of the fluid must be configured for a wide throughput range of the fluid and must have a highly dynamic configuration in order to function satisfactorily in every operating state of the combustion system. Such a metering system entails high costs, since it has to be adapted to the respective system.
It is accordingly an object of the invention to provide a method and a device for controlling a combustion system and for catalytic cleaning of exhaust gases, as well as a combustion system, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type, which are technically simpler and more cost-effective than the prior art, in which a fluid is added to the exhaust gas in a metered manner and which thus achieve a high precipitation rate of a noxious substance without an appreciable slip of a fluid or of a product of the fluid.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for controlling a combustion system and for catalytic removal from exhaust gas of a noxious substance generated in the combustion system, which comprises controlling combustion in the combustion system to adjust a constant desired value for a mass flow of the noxious substance or a desired value for the mass flow of the noxious substance linearly dependent on at least one variable characterizing an operational state; and metering an addition of a fluid to the exhaust gas to correspond to the desired value for the mass flow of the noxious substance.
The invention is based in this case on the concept that the mass flow rate of the noxious substance produced during the combustion is dependent to a high degree on the conditions, such as pressure and temperature, for example, prevailing during the combustion. The mass flow rate of the noxious substance is defined in this case as the throughflow rate of the noxious substance per time unit, measured in kg/sec. The combustion can accordingly be controlled in accordance with a prescribed desired value for the mass flow rate of the noxious substance. Of course, controlling the combustion in such a way also makes it possible to regulate the mass flow rate, i.e. to set the value of the mass flow rate within a certain tolerance range about a prescribed desired value. Since, in this way, the mass flow rate no longer changes within wide limits, particularly under changing operating conditions of the combustion system, the metering of the fluid which is necessary for the catalytic removal of the noxious substance from the exhaust gas can be substantially simplified. It is specifically configured only for a very small throughput range of the fluid in accordance with the desired value of the mass flow rate of noxious substance and optimized for this range. The highly dynamic configuration of the metering device is eliminated.
In accordance with another mode of the invention, the combustion can be controlled by influencing the air quantity fed to the combustion and/or the fuel quantity fed to the combustion. This controls the temperature prevailing during the combustion. That temperature constitutes a significant parameter for the quantity of noxious substances being produced.
It is known that during the combustion of a fossil fuel, a slight reduction in the temperature prevailing during the combustion results in a drastic reduction in the quantity of nitrogen being produced. In the case of an internal combustion engine, it is possible, in particular by adjusting the fuel/air mixture fed to the combustion, to bring about a change in the combustion temperature without incurring a reduction in performance in the process. In the case of an internal combustion engine, the fuel/air mixture can be adjusted in an easy mechanical manner through the use of a throttle flap located in the air intake. The fuel/air mixture can also be adjusted directly by electronic or mechanical measures using a carburetor.
In accordance with a further mode of the invention, in an internal combustion engine the combustion is controlled by influencing the ignition time and/or the feed or injection time of the fuel. The start of combustion can be moved in terms of timing in this way. Usually, the start of combustion is ahead of the so-called upper dead center i.e. the upper reversal point of the piston. If, on one hand, the start of combustion is moved forwards in terms of timing, the combustion takes place in a compression region, i.e. at a relatively high temperature and a relatively high pressure, which results in large amounts of nitrogen oxides being generated. If, on the other hand, the start of combustion is moved back in terms of timing, the combustion takes place in an expansion region, i.e. at a relatively low temperature and a relatively low pressure. Relatively few nitrogen oxides are generated. In modern engines, with injection equipment, such control is possible without a loss in performance and the same, or an improved, fuel consumption. On one hand, in an auto-ignition diesel engine, the combustion point can be adjusted solely by adjusting the time at which the fuel is fed in or injected. on the other hand, in an Otto or spark-ignition engine, the combustion is controlled by influencing the ignition time.
In terms of the configuration of the metering device for the introduction into the exhaust gas of the fluid which is necessary for the catalysis, it is advantageous if a constant desired value is prescribed for the mass flow of the noxious substance in the exhaust gas. In this case, the metering device can have a particularly simple configuration, and is specifically configured solely for a narrow throughput range.
In this way, an inexpensive series production of the metering device is possible. Such a regulating system is readily possible, for example, in modern engines with electronic engine management.
In terms of a combustion system for which it is not possible to control the mass flow of a noxious substance in accordance with a constant desired value due to the structural layout or to other peripheral conditions, it is advantageous if a desired value prescribed for the mass flow has a linear dependence on a variable which characterizes the operating state of the combustion system. Such a variable which characterizes the operating state can, for example, be the load or, in the case of an internal combustion engine, the engine speed. In the case of a combustion system with unregulated mass flow of the noxious substance to be removed, a small change in a variable characterizing the operating state, for example the temperature, can result in a large change in the mass flow. Therefore, a regulating system with a linearly adapted desired value already constitutes a significant improvement in terms of the configuration of the metering device. It is also conceivable for a desired value which is prescribed for the mass flow of the noxious substance to be removed, to have a linear dependence on a plurality of variables which characterize the operating state of the combustion system.
With the objects of the invention in view there is also provided, in a combustion system having an exhaust gas duct for conducting exhaust gas, a device for metered addition of a fluid to the exhaust gas, and a catalytic converter in the exhaust gas duct for converting a noxious substance by using the fluid, a device for controlling the combustion system and for catalytic removal of the noxious substance from the exhaust gas, comprising a device for determining a mass flow of the noxious substance; a control line connected to the combustion system; and a control unit for forming a constant desired value for the mass flow of the noxious substance or a desired value for the mass flow of the noxious substance linearly dependent on at least one variable characterizing an operational state of the combustion system, the control unit connected to the device for the metered addition of the fluid and to the device for determining the mass flow, the control unit having a terminal connected to the control line, and the control unit regulating the mass flow for controlling the combustion of the combustion system corresponding to the desired value for the mass flow and for a metered addition of the fluid corresponding to the mass flow.
In accordance with another feature of the invention, the device for determining the mass flow of the noxious substance is a measuring device for measuring the mass flow. The measuring device is disposed in the exhaust gas duct upstream of the catalytic converter in flow direction of the exhaust gas. However, it is also possible to store a characteristic diagram in an electronic memory. The mass flow of the noxious substance can be obtained from the characteristic diagram as a function of the respective operating state of the combustion system. In this case, the device for determining the mass flow includes a device for sensing the operating state, for example by measuring parameters which are relevant to operation, such as load or fuel consumption, the aforesaid electronic memory and a computer unit for reading out the characteristic diagram from the memory.
Such a device for cleaning exhaust gases can be connected directly to an existing exhaust gas line of a combustion system or integrated directly into the exhaust gas line.
The control unit may be provided with an electronic memory in order to store the determined measured values of the mass flow of noxious substances. In such a memory, the desired value for the mass flow, or in the case of a linearly dependent desired value, a prescribed characteristic curve or a prescribed characteristic diagram for calculating the desired value from one or more variables characterizing the operating state of the combustion system, may be stored. In order to control the combustion system, the control unit may also be equipped with a microchip, which calculates an appropriate control signal for the combustion system from the difference between the measured actual value of the mass flow and the desired value to be regulated, or obtains an appropriate control signal from a stored file. Depending on the regulated desired value, the control unit is also suitable for metering into the exhaust gas an appropriate quantity of the fluid which is necessary for the catalytic removal of the noxious substance.
In accordance with a further feature of the invention, the control line adjusts an air quantity and/or fuel quantity which can be fed to the combustion.
In accordance with an added feature of the invention, in terms of the cleaning of an exhaust gas of an internal combustion engine having a combustion which, as is known, occurs cyclically, the control line influences an ignition time and/or a feed time of the fuel. Such a control line can be constructed, for example, as an electronic control cable, which, in modern internal combustion engines, can be coupled directly to an existing piece of electronic injection equipment.
With the objects of the invention in view there is additionally provided a combustion system, in particular an internal combustion engine, having a device for controlling the combustion system and for the catalytic removal of a noxious substance contained in the exhaust gas, as described above.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and a device for controlling a combustion system and for catalytic cleaning of exhaust gases, as well as a combustion system, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.